The present invention relates to an impregnated capacitor comprising a roll of a metallized film impregnated with an insulating impregnating agent. Metallized film capacitors (hereinafter referred to as "MF capacitors") comprising a roll of a metallized film having a vacuum-deposited layer comprising aluminum or the like as an electrode have been used widely in the prior art. This is because the capacitor can be miniaturized in virtue of their excellent self-healing action which facilitates the increase of withstand voltage without insertion of another film between the electrodes. Films used for the production of the MF capacitors include a biaxially oriented polypropylene film (hereinafter referred to as OPP film). This film has been used widely, since it is more inexpensive than other films such as polyester film and it has preferable temperature dependence of dielectric loss.
The MF capacitors now used in practice are mainly so-called dry MF capacitors not impregnated with an insulating impregnating agent such as an insulating oil.
Generally, the potential gradients of not only the capacitors but also various electric appliances are increased when an insulating impregnating agent is present in the environment around the electrode or the conductor. In other words, the withstand voltage is improved advantageously under such a condition. The potential gradient is further increased by employing an impregnating agent having suitable electric properties.
Though the MF capacitors of impregnation type are preferred to those of dry type, the metallized films comprising a polypropylene film as the base film have the following disadvantage: when this film is impregnated with an insulating impregnating agent, the film is deformed or the impregnating agent penetrates between the vacuum-deposited metal layer and the base film bonded physically to each other to form cracks in the metal layer. In an extreme case, the metal layer is peeled off to cause a dielectric breakdown. Since the impregnating agent is applied after the film has been rolled up, the combination of the film and the impregnating agent should be selected suitably; otherwise the impregnating agent would not penetrate sufficiently into the film layers and no impregnation effects could be expected.
The dry MF capacitors have been improved to some extent by impregnating only an end side of the rolled up film and the outer periphery, taking advantage of the fact that the dielectric breakdown is caused at first in said end and outer periphery, leaving the inside thereof non-impregnated to form a semi-dry MF capacitor.
However, the improvement of the function of the semi-dry MF capacitor is limited, since the impregnation is effected only partially and the important surroundings of the electrode layer are not impregnated. Thus, as compared with the impregnated MF capacitors, the semi-dry MF capacitors cannot be said as satisfactory.
Under these circumstances various improvements of the impregnated MF capacitors have been proposed.
For example, Japanese Patent Laid Open No. 36972/1980 has disclosed an impregnated MF capacitor wherein the metallized film swollen with an impregnating agent has a degree of shrinkage of within 0.5%. Great Britain Pat. No. 1451499 has disclosed an oil-impregnated capacitor wherein the rate of length change of the polypropylene film due to the insulating oil at 80.degree. C. is up to 0.5% and the amount of diffusion of the insulating oil into the polypropylene film at 100.degree. C. is up to 10 wt. %.
However, the MF capacitors proposed heretofore are yet insufficient for practical use.
After investigations of the impregnated capacitor by impregnating the MF capacitor comprising the polypropylene film used as the base film with an impregnating agent, the inventors have found that no practical capacitor can be obtained by combining a specified impregnating agent with a polypropylene film selected according to values of dimensional stability of the impregnated film and amount of diffusion of the impregnating agent into the film as mentioned in the specifications of Japanese Patent Laid Open No. 36972/1980 and Great Britain Pat. No. 1451499.
After intensive investigations, the inventors have found surprisingly that it is necessary for obtaining a practical capacitor to combine the base film with the impregnating agent so that the ratio of the amount of the impregnating agent which penetrates between the base film layers to that of said agent which penetrates into the base film wound be within a specified range. Such a fact has not been known in the art.
When the impregnating agent penetrates into the metallized film in the step of impregnating the capacitor element of the roll of the metallized film, the dimension of the film is changed and the impregnating agent diffuses between the metal layer and the base film which are physically bonded to each other to cause the peeling or even dropping of the metal layer. Therefore, not only a high dimensional stability but also minimum penetration of the penetrating agent into the film is required in the penetrating step.
However, on the other hand, it is generally preferred that the impregnating agent is present around the electrode to improve the withstand voltage as mentioned above. In the MF capacitor, the electrode is surrounded by the film and the interlaminar gap. To obtain the sufficient effect of the impregnating agent, therefore, it is necessary that the capacitor element is impregnated with a large amount of the impregnating agent. Thus, the impregnating agent should penetrate into the interlaminar gap of the roll in an amount larger than that which penetrates into the film, since undesirable effects will result when the impregnating agent penetrates into the film in a large amount.
Namely, the penetration of only a small amount of the impregnating agent into the film is preferred from the viewpoint of protection of the vacuum-deposited metal layer, while the penetration of a large amount of said agent into the film and the interlaminar gap is preferred from the viewpoint of the effects of the impregnation.
The amount of the impregnation of the film varies depending on the molecular structure, molecular weight, viscosity, etc. of the impregnating agent. Further, the amount of the impregnation of a given impregnating agent varies also depending on many properties such as isotacticity index (I.I.) of the polypropylene used, degree of crystallization of the film surface which varies depending on a cooling process employed in the course of the preparation of the base film, roughness of the polypropylene film surface and corona discharge treatment.
Therefore, it is indispensable for selecting a suitable combination of the polypropylene base film with the impregnating agent to determine the amount of impregnation by a method which will be described below.
An object of the present invention is to provide a practically usable MF capacitor having a long life and capable of elevating the withstand voltage and contributing to the miniaturization.
The gist of the present invention resides in a capacitor wherein at least part of the capacitor elements comprises a metallized film comprising a polypropylene film as a base film and which is impregnated with an insulating impregnating agent comprising a compound having three condensed or non-condensed aromatic rings, the ratio of the amount of the impregnating agent which penetrates between the base film layers to that of said agent which penetrates into the base film per unit thickness (.mu.) as determined by the method described below being at least 60.